SLUA159 Datasheet, PDF (5/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
tude is insufficient to accommodate typical
input and output variations, The exact time is
obtained by solving the resonant capacitor
voltage equations for the condition when zero
voltage is attained.
The equation can be further simplified by
extracting the half cycle (180 degrees) of con-
duction which is a constant for a given resonant
frequency, and equal to ~rr/t+.
U-138
VCR (t)ve ^{LINE C H A N G E S}
The resonant component current (IcR = IL.)
is a cosine function between time tl and tz,
described as:
Fig. 7 -- Resonant Capacitor Voltage vs. Line
The absolute maximum duration for this
interval occurs when 270 degrees (3x/2& of
resonant operation is required to intersect the
zero voltage axis. This corresponds to the limit
of resonance as minimum load and maximum
line voltage are approached.
Contributions of line and load influences on
the resonant time interval t12 can be analyzed
individually as shown in Figs. 7 and 8.
Prior to time tl, the catch diode Do was not
conducting. Its voltage, VW, was linearly de-
creasing from V,N at time to to zero at tl while
input source v,Jv was supplying full output
current, IO. At time t,, however, this situation
changes as the resonant capacitor initiates
resonance, diverting the resonant inductor
current away from the output filter section.
Instantly, the output diode voltage, Vm, chang-
es polarity as it begins to conduct, supplement-
ing the decreasing resonant inductor current
with diode current I’, extracted from stored
energy in output inductor Lo. The diode cur-
rent waveshape follows a cosine function during
this interval, equalling IO minus IcR(t)#
Also occurring at time t,, the output filter
inductor Lo releases the stored energy required
Fig. 8 -- Resonant Capacitor Voltage vs. Load
to maintain a constant output current I,. Its
reverse voltage is clamped to the output voltage
V. minus the diode voltage drop V’ by the
convention followed by Figure 4.
Table III - RESONANT INTERVAL: tl - b
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